Project Summary Alzheimer's disease (AD) is the most common cause of dementia. AD is expected to increase in prevalence in the coming decades and poses a significant social and economic problem for an aging populace. Recently, a heterozygous mutation on the Triggering Receptor Expressed on Myeloid cells 2 (TREM2) gene was shown to confer one of the highest odds ratios for the development of AD. This mutation was proposed to cause an arginine to histidine (R47H) mutation resulting in loss of TREM2 function. Understanding the function of this TREM2 mutation is crucial to informing novel therapeutics to ameliorate AD. TREM2 is expressed on myeloid cells of the innate immune system, which include peripheral macrophages and resident CNS microglia, and therefore provide a novel target for immune-modulating therapies to treat AD. However, before therapeutics can be developed, the function of TREM2 must be determined. We previously found that TREM2 knockout AD mice had a significant reduction in amyloid plaque burden, neuroinflammation, and a decrease in hyperphosphorylated tau. We also observed a significantly loss of plaque-associated myeloid cells which were identified to be infiltrating macrophages. This was an unexpected finding, as TREM2 loss-of-function should augment AD pathology. This suggests that additional genetic models are needed to study the contribution of TREM2 to AD. In order to address this, we generated a novel TREM2 R47H knock in model using CRISPR/Cas9 genome editing technology. Our preliminary findings show that amyloid plaque number, plaque-associated myeloid cell number, and myeloid cell activation was significantly reduced. Again, these findings are contrary to the expected increase in AD risk. Therefore it is crucial to understand how the TREM2 R47H mutation modulates additional aspects of AD pathology. The goal of the current project is to characterize the myeloid cell component, amyloid homeostasis, and neurodegeneration in the TREM2 R47H AD model. This proposal will provide technical training in isolation of primary microglia for in vitro survival assays, flow cytometry to determine ontogeny of myeloid cells in the brain, and cell sorting to obtain RNA for transcriptome profiling for myeloid cell phenotype. Additionally, this proposal includes training in quantification of soluble vs. insoluble amyloid using ELISA and dot spot assays, and confocal imaging to quantify dystrophic neurites, hyperphosphorylated tau, and neuronal number. Finally, this proposal includes training in the assessment of deficits in spatial learning and memory using Morris water maze and working memory using radial arm water maze. The proposed experiments will test the hypothesis that TREM2 R47H may contribute to poor survival of resident microglia, poor infiltration of peripheral macrophages, or a combination of these processes. Ultimately, this may result in an increase in neurotoxic soluble amyloid, which may result in neurodegeneration and observable cognitive deficits. This proposal will contribute significant insight into the function of TREM2 such that novel therapeutics can be developed to treat of AD.